The present invention is directed to a cathode arrangement for the emission of electrons, a method for producing such a cathode arrangement and an electron beam source comprising such a cathode arrangement, and further to a vacuum treating arrangement with the said electron beam source.
In the electronic industry for the production of integrated circuitry as well as in the industry for producing hard layers or optical anti-reflex coatings, it is well-known to use electron beam evaporation processes. To achieve the high requirement with respect to homogenity of the coating thickness as well as to reach a high degree of reproductivity of the coatings, it is for such evaporation of high importance to provide for a thermically stable beam source system with a precise emission cathode for a reproducible cathode spot on a crucible of the evaporator. Thereby it is known to deflect the electron beam by about 270.degree. with the aid of an electron optical unit before the beam strikes the crucible. The position of the spot on the crucible is either open-loop or negative feedback, close-loop controlled. The distribution of energy over the evaporation spot and the overall energy over the evaporation surface of the crucible is also controlled. With respect to the heated cathode, thus the following criteria are of predominant importance:
high degree of material purity, PA1 long times of possible use, PA1 a satisfying space distribution of electron emission to ensure good electron-optical picturing with a high degree of reproductivity, PA1 short reaction times to respond to changes in energy by means of adjustment of the heater arrangement. PA1 Steady-state discharge test of coaxial LaB.sub.6 cathode of S. Tanaka et al.; Rev. Sci. Instrum. 59(1), January 1988, PA1 Directly heated lanthanum hexaboride filaments of K. N. Leung et al.; Rev. Sci. Instrum. 55(7), July 1984, PA1 Directly heated lanthanum hexaboride cathode of K. N. Leung et al.; Rev. Sci. Instrum. 57(7), July 1986.
Generally there are known two kinds of electron emission cathodes for said electron beam sources. In the first kind, the emission means are indirectly heated, i.e. the electron emitting part is provided in close proximity of, but separate from a heating arrangement.
Such a cathode arrangement is e.g. known from the article "Large area lanthanum molybdenum electron emitters" of D. M. Goebel et al, Rev. Sci. Instrum. 56(10), October 1985, American Institute of Physics, pages 1888 to 1893.
The advantage of such indirectly heated cathode arrangements is that the emitting surface is large. It is, on the other hand, a disadvantage that the construction is quite complicated to ensure the cathode heating, especially if there is considered that the heating arrangement must be dimensioned for a relatively high power in view of the distance between the emitting part and the heating arrangement therefor. The efficiency of such an arrangement is therefore poor because a substantial part of the heating energy is lost into the surroundings.
Further, such cathodes react slowly if a change of heating energy is applied for changing the emission energy of the beam, and such cathode arrangements are not suited for the use of emitting material with high operating temperatures as for tungsten.
A further kind of so-called indirect cathode heating, involves shooting electrons to the cathode which is to be used for the evaporation process, from behind. For this purpose, a secondary cathode is used with corresponding apparatus. In these cathode arrangements, the disadvantages mentioned above for cathode arrangements which are indirectly heated by heat radiation, do substantially not occur, but the efforts needed for construction are great. This latter procedure is primarily used for big area evaporator cathodes.
The second type of electron emission cathode is directly heated, in that the cathode is formed by a conductive wire which is heated by Joule heat. An electric current is passed through the wire and thus the wire is activated to emit electrons. Such a directly heated cathode arrangement as well as the above mentioned electron bombardment heating is e.g. known from "Elektronenstrahltechnologie" of Schiller, Heisig, Panzer, Wissenschaftliche Verlagsgesellschaft mbH, Stuttgart, 1977, pages 40 to 44.
The advantage of directly heated cathodes is the improved efficiency in that the heating energy is significantly better exploited compared with indirectly heated cathodes, and that further there is only needed a reduced constructional effort. A disadvantage is that for such known electron emitting cathodes with an electric wire, the Overall surface defined by such a wire is purely exploited for emission. Due to constructional limits the cathode wire may not be wound too tight to cover a predetermined area because a mutual contact of the wire segments must be prevented. The inherent elasticity of such wires and the problem to dispose parts of the wire as closely as possible one beside the other, result in the fact that within an overall area defined by such a wire cathode, a predominant part is occupied by gaps and only a smaller amount by the wire itself and thus there is only a smaller part which is in fact electron emitting.
Further, the usually circular cross-section of the wire emits electrons radially distributed to all sides with respect to the axis of the wire which leads to a disadvantageously diffused overall emission characteristic over the wire coil cathode with undesired characteristics for forming the beam.
From "Neutral-Beam Development", LBL, Berkeley, Calif. 94720, a heating helix, coated with LaB.sub.6 -crystals, is known, whereby the coating is heated as electron source with the help of the embedded helix.
In this context see also:
This technique is extremely complicated, especially if the thermical loading of the coating is considered. Here too, there is in fact provided an indirect heating, in that the emitting coating material is heated by the non-emitting heating helix. The same is valid for the electron emission cathode, known from the German laid open Patent no. 29 33 255 which is made of a ceramic material which is composed of LaB.sub.6 and of an electro-conductive material which latter is provided for heating purposes.